Dynamic compressor amplifier



M 1951 I E. s. PURINGTON 2,553,673

DYNAMIC COMPRESSOR AMPLIFIER Filed April 16, 1948 1; E llh 62% v sou Ml ELLISON s.' Foams-row.

(Ittomeg Patented May 22,1951

for the transient pulses.

formers.

UNITED STATES PATENT OFFICE;

DYNAMIC COMPRESSOR AMPLIFIER Ellison S. Purington, Gloucester, Mass., assignor to John Hays Hammond, Jr., Gloucester, Mass.

'in the output circuit of an electronic amplifier and has for an object to provide a circuit of the above type having novel and improved operat ing characteristics. 7

Another object is to provide a circuit of the above type having improved efiiciency of operation and in which the thump eifect and the effect of the control voltage upon the output is eliminated.

Various other objects and advantages will be apparent as the nature of the invention is more fully disclosed.

In a class A amplifier or in a dynamic expander amplifier, an electronic tube usually operates with considerable efiiciency for strongest signals, but in a dynamic compressor amplifier the electronic tube usually must operate at very low efficiency under all conditions. Thus in a compressor using a triode tube and grid bias control, the tube functions as a class A amplifier for very weak signals, but as the signals increase the grid is driven toward cutoff so that the tube gain, the

possible input signal and the output are highly reduced. The ratioof bias change to input grid swing may be so large that considerable care must be taken to prevent the output from containing serious transient pulses and low audio frequency signals caused by the bias changing voltage.

One method of minimizing this effect is to use two dynamic tubes in push-pull, with grid signals inserted out of phase by use for example of a transformer or phase inverter tube, and with the plates out of phase for the signal but in phase By using a push-pull output transformer, with either push-pull or single ended secondaries, the ratio of signal to transient disturbances may be greatly improved. Each tube, however, operates in substantially the same manner as before as regards grid input swing, and dynamic range.

In the present invention provision is made for utilizing two triodes in a single ended arrangement as distinguished from a push-pull arrangement, providing thump and transient'elimination without requiring input or output trans- The invention also provides for very greatly increased performance as regards grid input swing and dynamic range. The second of the two triodes is not used merely for providing thump reduction as in conventional push-pull circuits but makes its own important contribution to the compression operation. The functioning is somewhat analogous to that of a manu- Application April 16, 1948, Serial No. 21,375

3 Claims. (Cl. 179-471) ally controlled L pad attenuator inserted between a signal source and a signal load, with one tube representing the series arm of the L pad and increasing in internal impedance as the signal increases, and the other tube representing the shunt arm of the L pad and decreasing in internal impedance as the signal increases. In addition, the shunt arm of the pad is made of especially low impedance for strong signals by use of a feedback arrangement, thereby increasing greatly the amount of compression possible.

In accordance with the present invention the single-ended dynamic compressor amplifier includes a pair of triodes with plates and cathodes in parallel with a common plate feed and a common external output circuit. The input signal is impressed upon the grid of the first triode. Control voltages are derived from the output circuit by means of a rectifier network and are applied to the grid return paths of the two triodes in opposite sense so that an increased signal serves to decrease the conductance and plate current of the first triode, but increases the conductance and plate current of the second triode in such a manner that the total plate current drawn by the two tubes remains substantially constant. The decrease in conductance of the first tube thus reduces the amplification factor and results in a certain amount of dynamic compression. The increase in conductance of the second tube increases this effect by providing a shunt path of lower conductance across the output of the first tube. Hence the combined effect may be adjusted to provide the desired amount of dynamic compression without requiring either of the tubes to be overcontrolled. In addition, a feedback is taken from the common plate circuit of the tubes to the grid of the second tube in a sense to increase the range of effective internal impedance of the second tube for a given change of plate current. This feedback may have frequency selection characteristics if desired so that the compression may be varied for different frequencies as required for obtaining the proper acoustical eifect.

The novel features which are characteristic of this invention will be better understood by referring to the following description, taken in connection with the accompanying drawings in which a specific embodiment thereof has been set forth for purposes of illustration.

In the drawings the figure is a schematic diagram of a dynamic compressor system embodying the present invention.

In the embodiment shown in the drawing two assaws through capacitor 29 to the grid 36 of triode ill, v

and to ground respectively. The grid 30 of triode I is connected through resistor 32 from which it receives its audio, signal vgltageanda resistor 33 from which it receivesqits automatic control voltage, to the junction of resistors 22 and 23, so that in the absence of a control voltage, the grid 30 is positive with respect to ground dueto the voltage drop in resistor 23, but is not up to th ca h de nqis ti lnsl e, to. the b asi resistor 2 2. The grid 34 or tube l l is connected through the resistor 35 from whigh it receives its own audio signalvoltage and a resistor 38 from which it receives its automaticcontrol to the grounded endof resistor 23, so that in the absence of control voltage, the grid 34 of triode H is biased at ground and therefore more negatively than the grid 36 of triode Hi.

The currents in the control resistors 33 and 36, which are by-passedby smoothing and timing capacitors 3] and 38, are such that the control voltages,produced thereby increasethe negative bias on triode Iii anddecreasethe negative bias on triode ll. An increase in control voltages .thusicuts down, the gainoi the system by decreasing the gain of triodelii, and increasing the proportion of ,amplifiedoutput ofv triode In that flows through theshunting triode I I. The eirect is further increased by use of ajfeedback capaci- ..tor,.33 .connectedfromtheplate lead of triodes ill and H to theigrid3fi which is more efiectiye .to increase the shunting. effect as the effect itself becomes greater.

.i The plates l'zand. l3 of triodes I0 and II are connected through output blocking capacitor 44 to one, end of a volume control potentiometer 65, the otherend of whichis connected to a ground lead 46. Output. terminals 4 and 48 are connected between the. variable tap 49 of the potentiometer and the ground lead 43. The plate lead l orf triodes llland H is alsorconnected. through a .blockingandsignal selecting capacitortil to one ,end of the dynamic control potentiometer l, the :other end otwhichis connected to, the ground lead 46.

A variabletapiZ on the potentiometer 5| .is .connectedto the gridc55 of a driver triode 56, the cathode, 51 of-which .is connected tothe ground 1ead,-.46..thro,ugh a resistor 58 and .a bypass capacitor 6D,.and the,iplate-6l ofwhich is connected through feed resistor 62 and decoupling resistor 63-to the positive lead P5 of battery. It, ,and the junction of resistors 62 andfia is connected to ground through a bypass-capacitor 64. Il he platefil of rectifier driver -triodem56 is connected througha capacitor 65ito the anode of rectifier 66 which is also connected through a resistor. to the ground lead 45. The cathode of rectifier, is connected througha resistorBS, shunted by afilteringand timing capacitor 59 to the grQJ nd lead 4E, and also through a resistor Into the junction of resistorsjit and 36. Similarly the platefil. of the rectifier driver tu efic .connectedthrough a capacitor, ll to 4 the cathode of a rectifier 12, which is also corlnected through resistor 13 to the junction of resistors 22 and 23. The anode of rectifier I2 is connected through a resistor 14, shunted by a capacitor 75, to said junction of resistors 22 and 23, and also through a resistor 16 to the junction of resistors 32 and 33.

In operation, the conductance of tube I0 is large .and that oi tubell is. weal; for small input signals, that ,the system amplifies well especially because the feedback coupling 39 is ineffeclive to react upon the plate current of triode ll.

As the signal input increases, a part of the signal iiutputoi triodes ii and I l in parallel is impressed on the rectiuer, driver 56, which operates the rectif ers 66 vandjl'll and associated circuits in such a manner that the bias on triode i6 is increased I sand that on triode II is diminished, so that the amplification gain of the triodes Ill and II in parallel is dimimshedn The circuit may be so designed that the D. C. spa,ce currents of triodes ,llland ll addupto, the same amount regardless of the amount ,oi' rectified control, so that sudden changes in, the A. C. output of the amplifier do not cause pulses, of control signal in the output. The value of the rectifier driverdecoupling capacitor and, of the, setting of the potentiometer it! may be so chosen that the changesoi bias on triodes iii and ii are not excessive, or limiting devices may be used for this purpose. The entire variation of amplifier'gain may be seated in this single stage compressor, or the circuit shown may be I'QJ Ed b a imil r .q qu torpt o pressor stage.

Frequency characteristics by choice of the feedback coupling 39,,may provide a greater compressicn on some fl'fflquencies. than, on others. Thus it is within the scope of this invention to provide more compressionon low frequencies than on high through usev of a low pass, feedback in plac of capacitor 39. Frequency characteristics in thetcontrolcircuit may be established by suit- .able choices of constantsso that, the compression isin accordance with the acoustical value and not the electrical valueof the signal inithe output plates, cathodes and control grids, said plates being connected together and said cathodes being connected together to provide parallel space circuit paths for said tubes, a common output circuit connected to said plates, a common cathode impedance connected between saidcathodes and a point of fixed-reference voltage, a source of plate voltage having a positive and a negative terminal, .said negative terminal being connected to said point of fixed-reference voltage, a common feed resistor connected betweensaid positive terminal and-said plates, a-negative feedback coupling means connected between saidscommon output .circuit and the control grid of the-secondof said 76 being connectedrbetween, the control grid. of the first of said pair of electronic tubes and said intermediate point on said cathode impedance, the other of said networks being connected between the control grid of the second of said pair of electronic tubes and said point of fixed reference voltage, whereby said control voltage is applied to each of said electron tubes in opposite senses so that an increase in control voltage value decreases the conductance and plate current of the first tube but increases the conductance and plate current of the second tube and said tubes mutually cooperate to provide increased dynamic compression range and also to reduce the effect of said control voltages upon the common output circuit.

2. A dynamic compressor amplifier, as set forth in claim 1, in which said coupling means has fre- REFERENCES CITED The following references are of record in the 10 file of this patent:

UNITED STATES PATENTS Number Name Date 2,069,869 Yolles Feb. 9, 1937 2,251,594 Mayne Aug. 5, 1941 2,368,453 Deerhake Jan. 30, 1945 2,495,390 Shimek Jan. 24, 1950 

